Help! Giant Bubbles In Cookies!

Please help! I make cookies quite often using NFSC. Every once in a while I get giant bubbles/lumps in them when they are baking. I usually just discard the ones that have these lumps, but last night every cookie I made came out with HUGE lumps all over them. My husband made the dough for me, so I blame it on him! Ha! Does anyone know why this is happening? Thanks!

Thank you all! I gently pushed down the bubble in the batch I did tonight and it worked great. Unfortunately, that didn't help with the ones I did the night before. I couldn't even turn them over because the back had holes from where the bubbles rose up. I took a knife and scraped the bubbles off, almost like sanding them. I still wish I knew the reason why this happens, but at least I know how to fix it when it does!

I finally figured out what causes the many bubbles on the underside of my cookies. I generally reuse parchment sheets a few times. When i use a fresh sheet, the cookies are perfect; no spreading, no bubbles. After that, they start to spread a bit and then produce the bubbles. I'm assuming i'm getting the spread because of the residual grease left over from the previous baking. And, after baking the first time, i notice the parchment sheets wrinkle a bit so i'm wondering if that and the previous residue has anything to do with the bubbles. I would hate to use new sheets of parchment with every batch as it's not cheap, and i could be wrong, but that's what i'm noticing.